Popping out of first gear is most likely the sign of a worn out center bearing retainer plate. I don't believe the retainer is a serviceable item and therefore I think most rebuild shops just slap it on and reinstall it without shimming or replacing it with a heavier duty unit (like we would since we manufacture them). It could also be a worn shift hub or a wasted counter shaft bearing but you'd certainly have hoped those to have been properly inspected & serviced by a rebuilder. None of these can be fixed with external repairs such as our HD seat/socket and HD springs. They can help, but are more like a band-aid for a problem that will only get worse.
The stock center bearing retaining plate on a W56 should be fine. My W56b was a nice machined iron plate...........
